1. Field of the Invention
The present invention relates to a thermal transfer ink sheet containing a polyvinyl acetal having a specific structure, and to an ink cartridge, a coating composition for dye layer of thermal transfer ink sheet, and a thermal transfer recording method.
2. Background Art
These days, in particular, a color image-forming material is the mainstream of an image-recording material; and concretely, inkjet-type recording materials, thermal transfer-type image-recording materials, electrophotographic recording materials, transfer-type silver halide photosensitive materials, printing inks and recording pens are much used.
Thermal transfer recording includes a recording system where a thermal transfer material having a thermofusible ink layer formed on a support (base film) is heated with a thermal head to thereby melt the ink for recording on an image-recording material, and a recording system where a thermal transfer material having a thermal transferable dye-containing dye-donating layer formed on a base film is heated with a thermal head to thereby thermally diffuse and transfer the dye onto an image-receiving material. In the latter thermal transfer system, the dye transfer rate may be varied by changing the energy to be applied to the thermal head, therefore facilitating gradation recording, and the system is especially advantageous for high-quality full-color recording.
In the thermal diffusive transfer recording system, a dye-containing thermal transfer sheet (hereinafter this may be referred to as “ink sheet”) and a thermal transfer image-receiving sheet (hereinafter this may be referred to as “image-receiving sheet”) are put one upon another, and then the ink sheet is heated with a thermal head from which the heat generation is controlled by an electric signal applied thereto, whereby the dye in the ink sheet is transferred onto the image-receiving sheet to attain image information recording thereon. In the system, three colors of cyan, magenta and yellow may be transferred and recorded on the sheet, thereby producing a color image having a continuous color gradation.
An ink sheet polymer plays a role of holding a dye in the thermal transfer sheet, and polyester resins have heretofore been used for it. Polymer materials capable of forming color images of high density are reported in JP-A 4-45993 and JP-A 2002-307844. These days, however, the increase in the printing speed is extremely great, and the above resins could hardly satisfy both the requirement for the rapid transfer printing speed and the requirement for good photographic properties of high transfer sensitivity.